Polyolefins are versatile materials which are generally easily processed and useful in numerous applications. Historically, processors of polyolefins have needed to accept some undesirable properties along with their ease of processability. Such undesirable characteristics include high fractions of low molecular weight species leading to smoking during fabrication operations, high levels of extractable materials and the possibility of leaching of these low weight molecules out of the formed polymer articles or packaging. Over the years, polymers other than traditional low density polyethylene (LDPE) including materials such as linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) have been developed. While offering several beneficial properties, they have been accompanied by some of their own limitations including difficulty in processing, melt fracture tendencies and low melt strength.
The advent of single-site catalysis (SSC), particularly metallocene-type catalysis has offered the possibility of producing entirely new polymers with remarkably narrow molecular weight distributions (MWDs) or polydispersities. This means that some of the problems associated with the presence of very low molecular weight polymer species are virtually eliminated with polymers produced by these catalysts. Enhancements to the melt processability of these narrow MWD linear materials would add to the value of the materials for many end use applications. One of the methods which can enhance melt processability is the inclusion of long chain branching. We have found that the controlled inclusion of long branches (differentiated from short chain branches which result from the copolymerization of olefin comonomers) on an otherwise essentially linear backbone, produces significant changes in key Theological parameters, leading to enhanced melt processability. We have accomplished this in a manner which includes the ability to control overall polymer crystallinity and crystallization tendencies while offering additional points of accessible residual unsaturation. These may be left unaltered in the polymer resin, reduced by hydrogenation, functionalized, or utilized in post-formation curing to yield a material behaving much like a thermosetting polymer but having the benefit of processing like a traditional thermoplastic polyolefin.
In the art of polyolefin manufacturing, it is recognized that copolymerization of olefins (comonomers) in the polymer backbone will alter the crystallinity and therefore the density of the material by interfering with the ability of the polymer molecules to "pack." While such "short-chain branches" are effective in disrupting the crystal structure, thereby reducing density, they generally have little effect on the melt rheology of the polymers. For the purposes of describing this invention, we will discuss polymer molecular structure changes which are Theologically significant. Generally, this will include long-chain branching, or branches from the main polymer backbone which are longer than branches obtained by copolymerization of easily obtained, commercially available olefin monomers. Such rheologically significant branching will be noted in the behavior of the molten polymer: an enhancement of polymer melt strength, a reduced tendency for melt fracture, and an increase in viscous or flow energy of activation, E.sub.a. These Theological properties of the molten polymer are generally easily quantified and will provide a convenient method to distinguish polymers of this invention relative to the prior art. By contrast, attempts to directly quantify polymer long chain branches (e.g. by spectroscopic techniques) have a very limited range of applicability due to inherent limitations in the techniques.
These long-chain branches will generally enhance the melt-processability of polymers. This effect is particularly pronounced for polymers having narrow MWD, including those which are produced by single-site, specifically metallocene, catalysis. Such polymers having long-chain branching will generally have melt-flow properties enhanced for many applications (e.g., those applications benefiting from higher melt strength) than will like polymers without the long-chain branching.
The following publications address issues related to those outlined above; however, none have arrived at the same solution and offer the unique combination of properties of the present invention. The prior work is nonetheless significant, as discussed below.
DE 3240382 (Hoechst) refers to the use of small amounts of diolefins, including norbornadiene (see page 8) to control "verzweigung" (branching), density and elasticity.
EP 35242-B (BASF) discloses copolymerization of ethylene and alpha-omega (.alpha.,.omega.) diolefins to provide cross linked products.
EP 273654; EP 273655 and EP 275676 (Exxon) disclose copolymerization of dienes. Page 9, lines 33 to 37 of EP 275676 discusses the nature of incorporation.
U.S. Pat. No. 3,984,610 to Elston describes partially crystalline polymers of ethylene and .alpha.,.omega.-dienes or cyclic endomethylenic dienes containing at least one norbornene nucleus. The polymer apparently has long-chain branches derived from polymerization via the second unsaturation of the diene. This disclosure focuses on polymers with "low residual unsaturation." The limit is described, at page 3, line 33, as less than one carbon-carbon double bond per 1000 carbon atoms. Actually, the demonstration provided in columns 7 and 8 appears to show the greatest unsaturation to be 0.7 carbon-carbon double bond per 1000 carbon atoms, thus manifesting the apparent intent of the work being to provide truly low levels of residual unsaturation. By contrast, the polymers of the present invention generally have substantially higher levels of residual unsaturation, as illustrated in the Examples. This higher level of residual unsaturation provides enhanced opportunities for functionalizing or post-formation curing of molded/extruded articles, thereby providing a novel balance of melt processability and end-use properties.
U.S. Pat. No. 4,404,344 (EP 035 242) to Sinn describes the copolymerization of ethylene and alpha olefins or .alpha.,.omega.-dienes. Their description does not appear to contemplate the benefits of copolymerization of multiple mono-olefins with polyenes.
U.S. Pat. No. 4,668,834 (EP 223,394) to Rim, et al. describes low molecular weight copolymers of ethylene and an alpha olefin having three to twelve carbons. The polymer exhibits vinylidene (chain-end) unsaturation. These liquid polymers are useful in curable electrical potting compounds.
Kaminsky and Drogemuller described, in "Terpolymers of Ethylene, Propene and 1,5-Hexadiene Synthesized with Zirconocene/Methyl-aluminoxane," presented in Makromolecular Chemistry, Rapid Communications at 11, 89-94 (1990), the terpolymerization of 1,5-hexadiene with other olefins. The occurrence of long-chain branching was inferred by the authors. Not mentioned in this reference is our finding of the high propensity of 1,5-hexadiene to cyclize to a 5 membered cyclopentane-type ring structure, following 1,2 insertion into the chain. This feature makes 1,5-hexadiene a generally unattractive choice to initiate long chain branching, the bulky cyclic structures complicating chain flexibility and crystallizability. Diene moieties shorter or longer than 1,5 hexadiene are less prone to cyclize and consequently more attractive, as is shown later in the Examples.
Hoel describes, in U.S. Pat. No. 5,229,478 (EP 0 347 129), a process for producing elastomers of ethylene, propylene, and a diene with at least one internal double bond. In this manner, a readily processable rubber is easily made, such material being capable of curing after formation through cross-linking of the internal double bond. This description does not contemplate either dienes with two Z-N accessible double bonds or the benefits of using other alpha olefins for modification of crystallization and density.
U.S. Pat. No. 3,472,829 discloses an ethylene propylene norbornadiene terpolymer.
Canadian Patent 946,997 discloses an ethylene-propylene 1,4-hexadiene-1,7 octadiene tetrapolymer.
Japanese Patent B-70727/1991 discloses an ethylene-propylene 1,7 octadiene terpolymer obtained using a MgCl.sub.2 /TiCl.sub.4 --Al(i C.sub.4 H.sub.9).sub.3 catalyst. Additional disclosures include tetrapolymers formed from ethylene, propylene, 5-ethylidene-2-norbornene and 1,7-octadiene or 1,9-decadiene.
Incorporation of comonomers with ethylene has been known and practiced for years. Yano et al. describe, in EP 0446 013, a polyethylene, and its process for production, which has numerous regular methyl branches, or is copolymerized with propylene, along its backbone. This does not appear to provide any material rheological benefits.
Lai et al. provide a method of obtaining long-chain branching in U.S. Pat. No. 5,272,236 and U.S. Pat. No. 5,278,272 (WO 93/08221). These publications describe a system in which low monomer and high polymer concentrations are maintained to encourage what is described as long-chain branching. The quantification of the levels of long chain branching is via spectroscopic techniques and the long chain branching is reportedly independent of molecular weight distribution. There is no indication that the resulting polymers have enhanced levels of residual unsaturation.